TSref

As per list established by Victor Livakovsky:

For anyone, who will search for same topic, I've made a research by simple search of '$GLOBALS['TSFE']->register' ($TSFE->register gave 0 results) in content of all PHP files in TYPO3 4.4. Here is a list with some my explanations:

class.tx_cssstyled_content_pi1.php

function render_uploads($content,$conf) - Rendering the "Filelinks" type content element, called from TypoScript (tt_content.uploads.20):
$GLOBALS['TSFE']->register['ICON_REL_PATH'] - keeps relative path to file + filename
$GLOBALS['TSFE']->register['filename'] - self-explainable
$GLOBALS['TSFE']->register['path'] - self-explainable
$GLOBALS['TSFE']->register['fileSize'] - self-explainable
$GLOBALS['TSFE']->register['fileExtension'] - self-explainable
$GLOBALS['TSFE']->register['description'] - description for current file from 'imagecaption' field
$GLOBALS['TSFE']->register['linkedIcon'] - not sure about that, but I think is is bool value, defining, should file icon be linked or not
$GLOBALS['TSFE']->register['linkedLabel'] - same as above, but for label

function render_textpic($content, $conf) - Rendering the IMGTEXT content element, called from TypoScript (tt_content.textpic.20)
$GLOBALS['TSFE']->register['imageCount'] - count of all image in block
$GLOBALS['TSFE']->register['renderGlobalCaption'] - bool value, which defines, shoul a global caption be rendered below image block
$GLOBALS['TSFE']->register['IMAGE_NUM'] - number of current image in set (DEPRECATED)
$GLOBALS['TSFE']->register['IMAGE_NUM_CURRENT'] - number of current image in set (not deprecated :))
$GLOBALS['TSFE']->register['ORIG_FILENAME'] - path to the image + image name
$GLOBALS['TSFE']->register['imagewidth'] - self-explainable
$GLOBALS['TSFE']->register['imagespace'] - self-explainable
$GLOBALS['TSFE']->register['imageheight'] - self-explainable
$GLOBALS['TSFE']->register['rowwidth'] - width of whole block of images
$GLOBALS['TSFE']->register['rowWidthPlusTextMargin'] - same, as above + margin to the text
$GLOBALS['TSFE']->register['columnwidth'] - maximum possible width of image block + spaces at left and right side of block
$GLOBALS['TSFE']->register['totalwidth'] - not sure, but I guess it is same as $GLOBALS['TSFE']->register['rowwidth']

class.tslib_content.php

function IMGTEXT
$GLOBALS['TSFE']->register['IMAGE_NUM']
$GLOBALS['TSFE']->register['IMAGE_NUM_CURRENT']

function HMENU
$GLOBALS['TSFE']->register['count_HMENU'] - current number of a generated HMENU on a website
$GLOBALS['TSFE']->register['count_HMENU_MENUOBJ'] - number of current menu item for current HMENU object

function SEARCHRESULT
$GLOBALS['TSFE']->register['SWORD_PARAMS'] - search words

function TEMPLATE
$GLOBALS['TSFE']->register['SUBPART_'.$theKey] - contains html code of subpart, defined by $theKey

public function SWFOBJECT
$GLOBALS['TSFE']->register['MMSWFID'] - not sure, what it used for, but it holds: uniqid('mmswf');

public function QTOBJECT
$GLOBALS['TSFE']->register['MMQTID'] - uniqid('mmqt');

function lastChanged($tstamp) - Sets the SYS_LASTCHANGED timestamp
$GLOBALS['TSFE']->register['SYS_LASTCHANGED'] - timestamp of page (or content on current page) last modification

function splitObj($value, $conf) - Implements the "split" property of stdWrap; Splits a string based on a token (given in TypoScript properties), sets the "current" value to each part and then renders a content object pointer to by a number.
$GLOBALS['TSFE']->register['SPLIT_COUNT'] - current number of item in split

class.tslib_menu.php
function makeMenu() - Creates the menu in the internal variables, ready for output.
$GLOBALS['TSFE']->register['count_menuItems'] - number of all items in current menu

function subMenu($uid, $objSuffix='') - Creates a submenu level to the current level - if configured for.
$GLOBALS['TSFE']->register['count_menuItems'] - as I understood this register holds number of items not for the whole menu, but for current menu level

function writeMenu() - Traverses the result array of menu items configuration (made by generate()) and renders each item.
$GLOBALS['TSFE']->register['count_HMENU_MENUOBJ'] - number of current menu item for current HMENU object
$GLOBALS['TSFE']->register['count_MENUOBJ'] - number of current menu item for ALL HMENU objects on a page

Hope, that can help for someone after me :)